1. Field of the Invention
This invention relates to a gas sensor adapted for combustion control or the like of an internal combustion engine, and to a method for manufacturing the gas sensor.
2. Description of the Related Art
Gas sensors disposed in the exhaust system of an internal combustion engine for detecting oxygen concentration in exhaust gas are utilized for combustion control of an internal combustion engine. Specifically, a known gas sensor element included in the gas sensor is equipped with: a solid electrolyte layer made of a ceramic such as zirconia having oxygen ion conductivity; a detection electrode disposed on one surface of the solid electrolyte layer and made of platinum or the like for contacting a gas to be measured; and a reference electrode disposed on the other surface of the solid electrolyte layer and made of platinum or the like for contacting a reference gas. On the surface of the detection electrode, moreover, a porous electrode-protecting layer made of alumina-magnesia-spinel or the like is formed to prevent poisoning of the detection electrode.
This gas sensor is a so-called “oxygen-concentration electromotive force type” gas sensor which generates an electromotive force in accordance with the oxygen concentration in the gas to be measured.
There is a need for a gas sensor which more precisely controls combustion of, an internal combustion engine so as to comply with recent, more stringent regulations on exhaust gas. A gas sensor element which satisfies this need must exhibit less deviation in λ point from a theoretical one and must precisely measure oxygen concentration. In a conventional gas sensor, however, the measurement precision of oxygen concentration may be impaired depending on the kind or the like of the exhaust gas. For example, hydrogen present in the exhaust gas has a high diffusion rate (the rate at which the exhaust gas reaches the detection electrode through the electrode-protecting layer) and can reach the detection electrode more easily than other components in the exhaust gas. The first diffusing hydrogen reacts with the detection electrode, and the detection electrode may mistakenly determine λ point which has deviated from the theoretical one to make precise combustion control difficult.
In order to provide a gas sensor having a small λ point deviation and which is capable of controlling precise combustion of the internal combustion engine, JP-A-11-237361 proposes a gas sensor having a catalyst layer containing a catalytic Pt metal or the like which exhibits excellent catalytic performance while covering the electrode-protecting layer. As a result, the hydrogen in the exhaust gas reacts with the catalyst layer to suppress diffusion of hydrogen to the detection electrode, to thereby provide a gas sensor which can reduce λ point deviation and which can precisely control combustion of the internal combustion engine.
Also known is a gas sensor (as proposed in JP-A-8-7177) having a catalyst layer containing catalytic metal which covers a catalytic metal-containing electrode-protecting layer. In this gas sensor, more catalytic metal is present in the catalyst layer than in the electrode-protecting layer, so that the hydrogen reacts in the catalyst layer (as disclosed in JP-A-11-237361). Thus, diffusion of hydrogen to the detection electrode can be suppressed so as to reduce λ point deviation and thereby provide a gas sensor which can precisely control combustion of the internal combustion engine.
3. Problems to be Solved by the Invention
However, the catalyst layer of JP-A-8-7177 is generally formed by preparing a slurry containing a material for forming the catalyst layer and by applying the slurry to cover the electrode-protecting layer. Therefore, the catalyst layer is not chemically bonded to the electrode-protecting layer, and may peel off of the electrode-protecting layer. The electrode-protecting layer is then directly exposed to exhaust gas containing hydrogen. The fast diffusing hydrogen may contact the detection electrode so as to introduce a deviation error into the intrinsically detected λ point and thus make precise combustion control difficult.
According to JP-A-8-7177 similar to JP-A-11-237361, on the other hand, even if the catalyst layer peels off of the electrode-protecting layer, the electrode-protecting layer itself contains catalytic metal. As a result, the catalytic metal in the electrode-protecting layer can react with the hydrogen to suppress diffusion of hydrogen to the detection electrode. Generally, the electrode-protecting layer is formed to cover the detection electrode by a plasma-spray coating so that contact is more intimate therebetween than between the electrode-protecting layer and the catalyst layer.
However, the unburned contents (e.g., CO or the like) in the exhaust gas may be adsorbed by or may react with the catalytic metal carried by the electrode-protecting layer, and the catalytic metal may expand. When the catalytic metal is dispersed in and carried by the electrode-protecting layer, the catalytic metal in the electrode-protecting layer near the detection electrode side may expand and peel off of the detection electrode. The detection electrode is then directly exposed to exhaust gas containing hydrogen. As a result, the presence of hydrogen can introduce a deviation of the λ point detected by the defection electrode from the theoretical one and thus make precise combustion control difficult.